1. Field of the Invention
This invention relates to apparatus and methods for the manufacturing of functional structures on dimensionally unstable substrates by direct-write printing techniques, in particular the manufacturing of electronic devices on flexible, plastic substrates.
2. Description of the Related Art
Integration of functional, electronic devices onto flexible substrates will potentially enable new electronic products such as flexible displays, or low-cost intelligent labels, as well as new manufacturing approaches, such as roll-to-roll manufacturing. However, many flexible substrates, such as plastic substrates, exhibit significant dimensional changes when subject to mechanical stress or temperature variations both of which occur during any manufacturing process. When a plastic substrate such as a thin sheet of polyethyleneterephtalate (PET) is heated it tends to shrink, and therefore the dimensions of any pattern which has been defined on the substrate is changing as a result of the heat exposure. Given an arbitrary reference point on such a pattern one can define an absolute distortion field, which attaches to every feature on the substrate a distortion vector equal to the difference vector of the real position of the feature on the substrate with respect to the reference point and its nominal position that was intended when the pattern was defined. The nominal position can, for example, be determined by a pattern on a photomask in the case of a lithographic patterning process, or by a software instruction file which moves the positioning stage of a materials deposition unit such as a printhead in a direct-write patterning system. For many low-cost, flexible substrates such as PET or polyethylenenaphtalate (PEN) typical absolute distortions on a substrate with dimensions of 12-14″ are on the order of 50-100 μm. Such distortions cause severe problems in a manufacturing process which requires definition of multiple patterns on top of each other with good registration of the features of an upper pattern to the features of a previously defined pattern. In the case of a conventional lithographic patterning process with multiple mask levels it is very difficult to compensate for this distortion, particularly if the distortion pattern is not the same every time the process is performed, but changes when environmental conditions, process conditions, or materials properties vary.
Direct-write printing uses a materials deposition unit/printhead which can be positioned above the substrate to deliver a defined-amount of functional material into well-defined locations, and is a useful approach to manufacturing of electronic devices. This technique is capable of overcoming distortion problems because of its inherent ability to compensate for the distortion by first detecting the distortion pattern on the substrate, and then modifying the positioning instructions for the printhead in order to compensate for the distortion and deliver the material in accurate registration with any previously defined pattern on the substrate. The process of detection can either be performed by measuring the distortion pattern prior to the materials deposition step, or by detecting the local distortion and correcting for it on the fly.
Some distortion correction schemes have been described in the applicant's earlier patent application US 2003/0059984. Examples of direct-write printing techniques to which such distortion compensation schemes are applicable are, but not limited to, inkjet printing, direct-write laser patterning/ablation, or dispensing. Distortion compensation schemes can also be implemented in certain adaptive lithography techniques such as, for example, described Zemel, et al., Proceedings of Printed Circuit Expo IPC, Long Beach, Calif., Mar. 26-28, 2002.
In order to achieve high-throughput manufacturing the materials deposition unit needs to have many parallel channels. In the case of an inkjet printhead each heads comprises a large number (several 100's) of nozzles, which can be operated independently from each other, and a high-volume manufacturing inkjet printer will need to have several individual printheads mounted together. The nozzles on a printhead are typically arranged in a linear array with a well defined nozzle pitch d on the order 250 μm. For example, in order to achieve a minimum TACT, the printer should ideally be able to define an arbitrary pattern on the substrate in a single pass. In the direction perpendicular to the print direction several printheads may be mounted in order to cover the full width of the substrate.
In such a multiple head assembly complex distortion compensation algorithms can be employed in order to ensure that an arbitrary substrate distortion can be compensated for. One scheme is illustrated in FIG. 1. N printheads are mounted behind each other each of which is shifted by a defined distance d/N perpendicular to the print direction. In the ideal case in which the substrate is undistorted and the pitch of the pattern to be printed is equal to the nozzle pitch (if the latter is not the case the head can be raked by a certain angle to achieve the desired pattern pitch), the pattern can be defined entirely by firing the nozzles of one print head only. If the substrate is distorted and the printed pattern needs to compensate for this distortion each portion of the pattern can be printed with that nozzle on a particular printhead which comes closest to the required position (FIG. 1), when moving the print head assembly along the print direction. If N is chosen such that d/N is smaller than the required registration accuracy an arbitrary distortion pattern can be compensated for in this way.
A significant drawback of this scheme is that it requires a large number of printheads, and more importantly, a large number of nozzles will not be firing regularly, but will remain idle for significant periods of time. In the case of inkjet printing, drying of the ink on the nozzle plate is a serious problem, which can cause degradation of the directionality of the droplets emitted from the nozzle, or in the worst case, nozzle blockage. In order to avoid such problems it is desirable that all nozzles are in use regularly, and that no nozzle remains idle for prolonged periods of time. In the distortion compensation scheme of FIG. 1 this requirement is difficult to meet.
It is known, from US 2002/0105688, to provide a printer with a plurality of heads, each rotatable and slidable, with control of nozzle row angle and nozzle row spacing, for fabricating an electroluminescent device. Reference is particularly made to FIG. 4 of this application and the text at paragraphs 143-147. A technique for distortion compensation in a hand-held ink jet printer is described in U.S. Pat. No. 5,593,236 and U.S. Pat. No. 5,988,900 in which, broadly speaking, rollers are used to determine a position of the print head with respect to the medium upon which the head is to print. Compensation for distortion caused by mechanical image stretch, in particular uni-directional stretch caused by transport of paper through an ink jet printer, is described in US 2004/0155948.